New deep setting longline technique for bycatch mitigation.
A typical pelagic tuna longline catches deep as well as shallow dwelling fish. Unfortunately, many unwanted species are often caught on the shallowest hooks of a longline, even if the longline is targeting deeper species. Attempts have been made in the past to adjust the depth of set to capitalise on the knowledge that the most sought after species are caught in deeper water. Attempts have also been made to avoid the unwanted species that dwell in shallower waters. This has been accomplished in some areas by banning shallow setting. Unfortunately, most attempts to target only deep dwelling species also have a high percentage of their gear in shallow waters, or have technical problems that make them unsuitable and unlikely to be adopted by longline fishermen. This report outlines a new deep setting technique that lands all of the hooks in a longline below a critical depth, out of range of most bycatch species, and down where the most sought after target species are usually captured. The method is simple enough to be adopted by almost any longline operation. Indications during fishing trials were that target species catch rates could be enhanced using this new deep setting technique.